1. Field of the Invention
Load additives protect metal surfaces of gears and bearings against uncontrollable wear and welding as moving parts are heavily loaded or subjected to high temperatures. Incorporating high load-carrying capacity into a premium quality turbo oil without adversely impacting other properties can significantly increase the service life and reliability of the turbine engines.
The mechanism by which load additives function entails an initial molecular adsorption on metal surfaces followed by a chemical reaction with the metal to form a sacrificial barrier exhibiting reduced friction between the rubbing metal surfaces. In the viewpoint of this action, the effectiveness as load-carrying agent is determined by the surface activity imparted by a polar functionality of a load additive, and its chemical reactivity toward the metal; these features can lead to a severe corrosion if not controlled or prevented until extreme pressure conditions prevail. As a result, the most effective load additives carry deleterious side effects on other key turbo oil performances: e.g., corrosion, increased deposit forming tendency and elastomer incompatibility.
2. Description of the Prior Art
U.S. Pat. No. 4,140,643 discloses nitrogen- and sulfur-containing compositions that are prepared by reacting a 2,5-dimercapto-1,3,4-thiadiazole (DMTD) with oil-soluble dispersant and subsequently reacting the intermediate thus formed with carboxylic acid or anhydride containing up to 10 carbon atoms having at least one olefinic bond. The resulting compositions are claimed to be useful in lubricants as dispersant, load-carrying additive, corrosion inhibitor, and inhibitors of Cu corrosivity and lead paint deposition.
U.S. Pat. No. 5,055,584 discloses maleic derivative of DMTD to be used as antiwear and antioxidant in lubricating composition.
U.S. Pat. No. 4,193,882 is directed to improved corrosion inhibiting lube composition that contains the reaction product of DMTD with oleic acid.
Other references which teach the use of DMTD derivatives in lube composition to improve one or several of performance features (antiwear, extreme pressure, corrosion inhibition, antioxidancy) are EP 310 366-B1, U.S. Pat. No. 2,836,564, U.S. Pat. No. 5,126,396, U.S. Pat. No. 5,205,945, U.S. Pat. No. 5,177,212 and U.S. Pat. No. 5,279,751.
EP 434,464 is directed to lube composition or additive concentrate comprising metal-free antiwear and load-carrying additives containing sulfur and/or phosphorous, and an amino-succinate ester corrosion inhibitor. The antiwear and load additives include mono- or di-hydrocarbyl phosphate or phosphite with the alkyl radical containing up to C.sub.12 or an amine salt of such a compound or a mixture of these; or mono- or dihydrocarbyl thiophosphate where the hydrocarbon (HC) radical is aryl, alkylaryl, arylalkyl, or alkyl or an amine salt thereof; or trihydrocarbyl dithiophosphate in which each HC radical is aromatic, alkylaromatic, or aliphatic; or amine salt of phosphorothioic acid; optionally with a dialkyl polysulfide and/or a sulfurized fatty acid ester.
U.S. Pat. No. 4,130,494 discloses a synthetic ester lubricant composition containing ammonium phosphate ester and ammonium organo-sulfonate, especially useful as aircraft turbine lubricants. The afore-mentioned lubricant composition have good extreme pressure properties and good compatibility with silicone elastomers.
U.S. Pat. No. 3,859,218 is directed to high pressure lube compositions comprising a major portion of synthetic ester and a minor portion of load-bearing additive. The load-carrying additive package contains a mixture of a quarternary ammonium salt of mono-(C.sub.1 -C.sub.4) alkyl dihydrogen phosphate and a quarternary ammonium salt of di-(C.sub.1 -C.sub.4) alkyl monohydrogen phosphate. In addition to the improved high pressure and wear resistance, the lubricant provides better corrosion resistance and causes less swelling of silicone rubbers than known oils containing amine salts of phosphoric and thiophosphoric acids.